[sdiy] sort of OT: The virtues of solid state bridge rectification vs. tube half-wave rectification

[Roy J. Tellason]

On Tuesday 04 December 2007 17:35, anthony wrote:
> I have a nice, big HV transformer from a piece of tube gear that used solid
> state rectification of the full-wave bridge type. The HV secondary is 300V
> with no center tap. I'm sure it can handle a lot of current because it
> served as the B+ supply to about 20 tubes, 4 of them EL84's pulling voltage
> regulator duty, but mostly
> twin-triodes.
>
> I am planning on using it in a push-pull amp using 2 EL34's that I want to
> bias purely Class A. I always forget how to figure out what the final
> rectified and filtered output would be based 9on the P-P or RMS voltage,
> whatever. I had a thought that bridge rectifying the 300VAC would give me a
> voltage too high to run the amp purely in Class A. But then I haven't
> really run the numbers on EL34's, except to figure that one of the filament
> windings (there are 2 6VAC filament windings with no center taps) will work
> for 2 EL34's if it worked for 4 EL84's.
>
> Anyway, now comes my second idea: I have this big 9-pin (noval? whatever
> the non-mini 9-pin base is...) damper rectifier, a 6DW4B, whose specs seem
> stout enough (unlike the 3A3 that I looked up too). Wouldn't the half-wave
> rectified voltage ultimately be lower than with a solid state bridge?

Yes.  There will be a voltage drop across the tube,  typical numbers for those 
designed as LV rectifiers being in the range of tens of volts somewhere (look 
up 5U4 and 5Y3 and such to get an idea).

And as far as "specs seem stout enough" the numbers can be deceptive.  I 
looked that tube up just now,  and also looked up the 5Y3,  just for 
comparison.  There's a rather interesting chart on the second page of the 5Y3 
datasheet,  one that you might want to look at.  I remember that chart from 
my times of perusing actual tube manuals.  And that tube is rated for higher 
current output than the 6DW4 is.

> I've run a few configurations on Duncan Amps' PSU designer software and
> this does seem to be the case. I think I'll need a separate filament
> transformer for the heater on this though, since one winding will be just
> for EL34's and the other one for the 12AX7's.
>
> So I suppose my question is: is there really any mojo in using a tube
> rectifier in a more or less 100% Class A push-pull amp. I remember reading
> something somewhere to this effect, but I can't remember what.

The one good thing about tube rectifiers is that you're not applying full B+ 
to the rest of the tubes in there before they're heated to operating 
temperature.  Trying to pull that much current through them while they're 
either cold or in the process of warming up can strip cathode material right 
off.  If you want to use a solid-state rectifier you'll have a "stiffer" 
supply,  perhaps with a bit higher output voltage,  but I'd put a "standby" 
switch in there to keep from applying the B+ to the rest of the unit until 
the tubes had warmed up.

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